Hazrat Abu Huraira (may Allah be pleased with him) says that the Noble Prophet (peace and blessings be upon him) would spread his hands in supplication in such a way that I could see the whiteness of his blessed armpits. The narrator says that in my opinion, this was on the occasion of the prayer for rain (Salat al-Istisqa).
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 8830